The act of switching radio frequency signals in an integrated circuit is carried out by a Radio Frequency (RF) switch circuit. RF switches are well known in the art and provide a key building block in wireless systems. RF switches may be utilised in numerous applications such as mobile phones and wireless Local Area Networks (LANs). Such switches may include any number of switching elements which cooperate to control the flow of RF power between various circuit nodes. Performance metrics such as low insertion loss, high linearity, switching time, high isolation and power handling are critical in RF switch design.
Generally an RF switch does not consist of the RF switching circuit alone. Typically a RF switch system is comprised of two domains; an RF domain which includes the switching elements and a Direct Current (DC) domain which includes control logic, bias generation and power management circuitry. When the switch is operational, a high degree of isolation must be maintained between the RF and DC domains. Inadequate isolation between domains will compromise performance of the switch resulting in increased loss, reduced linearity, reduced power handling capability, reliability and operating lifetime may also be reduced.
The DC domain section of an integrated RF switch may include a negative voltage generator which typically consists of an oscillator, clock buffering and a switched capacitor charge pump. Full scale CMOS clock levels exist within the negative voltage generator resulting in the presence of tones at the oscillator fundamental frequency and its harmonics within the switch. The oscillator fundamental frequency is normally in the 1-10 MHz range where the upper limit is set by the requirement to restrict the frequency of large amplitude tones and the lower limit is set by the requirement to limit the voltage ripple on the switched capacitor charge pump output.
The presence of tones due to the negative voltage generator can limit the overall linearity and spurious output of the RF switch. Tones from the negative voltage generator can feed directly from the positive and negative supply references of the switch drivers through to the gate or body terminals of switching element transistors to the RF ports. The presence of tones from the negative voltage generator at the gate or body terminals of the switching element transistors may also result in the generation of intermodulation product tones when mixing occurs with an applied RF signal within switching element transistors. Mixing of tones from the negative voltage generator with the applied RF fundamental tone may also occur at circuit elements within the DC domain section because some of applied RF signal will couple into the DC domain section through the power supply, non-zero impedance on the ground connections, routing traces or other paths. Intermodulation product tones in the DC domain section can propagate to the switching element transistors limiting the linearity performance of RF switching element transistors.
Isolation filters are typically provided between the DC and RF domain sections on the path of the signals controlling the RF switching element transistors to limit the amplitude of spurious tones due to the presence of a switching CMOS clock signal in the DC section that may propagate into the RF domain section. The cut-off frequency of the low pass filter between the DC and RF domain sections is limited by the resistance value in the filter that may be tolerated before the switching time is impacted. The resistance value restriction in turn limits the attenuation of spurious tones that can be achieved by filters in conventional RF switch designs.
In applications where low spurious performance is required an external negative supply may have to be used to avoid spurious tones from the on-chip negative voltage generator. Switching time is a measure of how quickly a switch path can be turned on, i.e., transition from a high isolation off-state to a low impedance on-state, or how quickly a switch path can be turned off, i.e., transition from low impedance on-state to high isolation off-state. The switching element transistors provide a capacitive load to the drivers that in conjunction with the overall effective resistance on the control signal path to the switching element transistor produce an RC-time constant that limits how quickly the control voltage on the gate or body terminals may be adjusted. The switching time may be specified as the time from 50% transition on an external control pin to the time when an RF power signal at an output of a selected path reaches 90% of its final value for off-on transition. In other words, the switching time may be specified as the time from a 50% transition on an external control pin to a moment in time when an RF power signal at an output of a selected path reaches 10% of its final value for on-off transition. Targets for switching time may range from under 100 ns to 10 μs depending on the application.
There is therefore a need to provide an RF switch which addresses at least some of the drawbacks of the prior art.